Efficiency of integrated waveguide probes in the detection of fluorescence and backscattered light

Integrated optical probes for detecting backscattered light in, e.g., Raman spectroscopy show desirable characteristics compared to conventional optical fiber probes, although the latter ones may have better collection efficiency in many cases. Major advantages of integrated probes include reduced size; reduced background noise due to scattering in the probe because of reduced propagation length; potential for monolithic integration with filters and spectrometers; very small collection volume, providing high spatial resolution; and polarization maintenance. We demonstrate that when scattered light needs to be collected from a thin layer close to the probe surface, integrated probes can have better collection efficiency than fiber probes do. We modeled a multimode integrated waveguide probe by adapting an analytical model that had been developed for fiber probes. The model was extended in order to account for arbitrary waveguide geometries and a low number of discrete waveguide modes compared to the quasi-continuum of modes in a typical multimode fiber. Using this model we compared the collection efficiencies of integrated and fiber probes for a thin scattering sample. We found that the integrated probe has a higher collection efficiency for scattering layer thickness and probe-to-layer distance both smaller than ~100 um.